Abstract Thermal stability of charged LiCoO 2 cathodes with various surface areas of active material is investigated in order to quantify the effect of LiCoO 2 surface area on thermal stability of cathode. Thermogravimetric analyses and calorimetry have been conducted on charged cathodes with different active material surface areas. Besides reduced thermal stability, high surface area also changes the active material decomposition reaction and induces side reactions with additives. Thermal analyses of LiCoO 2 delithiated chemically without any additives or with a single additive have been conducted to elaborate the effect of particle size on side reactions. Stability of cathode–electrolyte system has been investigated by accelerating rate calorimetry (ARC). Arrhenius activation energy of cathode decomposition has been calculated as function of conversion at different surface area of active material.